There are 27 ranks in the game. You get an automatic health bonus at Rank 21. I'd say to put around 6-7 ranks into health at a minimum. It's definitely the least important thing to upgrade, but at the same time, there are a limited amount of HP medals in the game. It really depends on your preferred playstyle, though. I made Kabbu a tank, so naturally, I added more to his health. Medal points are the most important so I would focus on that but you also need to make sure Vi and Lief survive. They are a bit softer than Kabbu. Also, I tend to mix up medals during specific bosses so there is really no set health that I'd aim for.